Although many cellular actions of 5-HT have been described, consequences of these actions for behavior have been difficult to assess. Additionally, many studies have characterized relatively long lasting effects of serotonin that are important for processes related to learning and memory. In contrast, this study focuses on the B21/B8 synapse, a sensory-motor connection that is gated in that afferent activity is transmitted in a phasic manner that is controlled by an ongoing CPG. Experiments proposed will determine how this connection can be modulated by MCC activity without disrupting essential features of sensory-motor transmission. How MCC activity can up-modulate afferent activity in B21 and/or the transmission of this activity to B8 without disrupting the "gated" nature of the connection, and without increasing the excitability of B21 to the point where B21 no longer responds to changes in the characteristics of a peripheral stimulus, will be determined. Whether the MCCs accomplish this by exerting effects at a few key locations or whether effects of MCC activity are widespread will be determined. In summary, the MCCs, a pair of modulatory neurons whose contribution to behavior has been studied in intact animals, will be studied. How MCC activity alters transmission at a sensory-motor connection that is also experimentally advantageous in that it can be studied in semi-intact feeding preparations, as well as in reduced preparations, will be determined. In this study, therefore, insights will be gained both into mechanisms of action of serotonin, and physiological consequences of these actions for behavior.